﻿// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information
// 
// Licensed to the Apache Software Foundation (ASF) under one or more
// contributor license agreements. See the NOTICE file distributed with
// this work for additional information regarding copyright ownership.
// The ASF licenses this file to you under the Apache License, Version 2.0
// (the "License"); you may not use this file except in compliance with
// the License. You may obtain a copy of the License at
// 
// http://www.apache.org/licenses/LICENSE-2.0
// 
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// 

using System;
using System.Collections;

namespace log4net.Util
{
    /// <summary>
    /// An always empty <see cref="IDictionary"/>.
    /// </summary>
    /// <remarks>
    /// <para>
    /// A singleton implementation of the <see cref="IDictionary"/>
    /// interface that always represents an empty collection.
    /// </para>
    /// </remarks>
    /// <author>Nicko Cadell</author>
    /// <author>Gert Driesen</author>
#if !NETCF
    [Serializable]
#endif
    public sealed class EmptyDictionary : IDictionary
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="EmptyDictionary" /> class. 
        /// </summary>
        /// <remarks>
        /// <para>
        /// Uses a private access modifier to enforce the singleton pattern.
        /// </para>
        /// </remarks>
        private EmptyDictionary()
        {
        }

        /// <summary>
        /// Gets the singleton instance of the <see cref="EmptyDictionary" />.
        /// </summary>
        /// <returns>The singleton instance of the <see cref="EmptyDictionary" />.</returns>
        /// <remarks>
        /// <para>
        /// Gets the singleton instance of the <see cref="EmptyDictionary" />.
        /// </para>
        /// </remarks>
        public static EmptyDictionary Instance
        {
            get { return s_instance; }
        }

        /// <summary>
        /// Copies the elements of the <see cref="ICollection"/> to an 
        /// <see cref="Array"/>, starting at a particular Array index.
        /// </summary>
        /// <param name="array">The one-dimensional <see cref="Array"/> 
        /// that is the destination of the elements copied from 
        /// <see cref="ICollection"/>. The Array must have zero-based 
        /// indexing.</param>
        /// <param name="index">The zero-based index in array at which 
        /// copying begins.</param>
        /// <remarks>
        /// <para>
        /// As the collection is empty no values are copied into the array.
        /// </para>
        /// </remarks>
        public void CopyTo(Array array, int index)
        {
            // copy nothing
        }

        /// <summary>
        /// Gets a value indicating if access to the <see cref="ICollection"/> is synchronized (thread-safe).
        /// </summary>
        /// <value>
        /// <b>true</b> if access to the <see cref="ICollection"/> is synchronized (thread-safe); otherwise, <b>false</b>.
        /// </value>
        /// <remarks>
        /// <para>
        /// For the <see cref="EmptyCollection"/> this property is always <b>true</b>.
        /// </para>
        /// </remarks>
        public bool IsSynchronized
        {
            get	{ return true; }
        }

        /// <summary>
        /// Gets the number of elements contained in the <see cref="ICollection"/>
        /// </summary>
        /// <value>
        /// The number of elements contained in the <see cref="ICollection"/>.
        /// </value>
        /// <remarks>
        /// <para>
        /// As the collection is empty the <see cref="Count"/> is always <c>0</c>.
        /// </para>
        /// </remarks>
        public int Count
        {
            get { return 0; }
        }

        /// <summary>
        /// Gets an object that can be used to synchronize access to the <see cref="ICollection"/>.
        /// </summary>
        /// <value>
        /// An object that can be used to synchronize access to the <see cref="ICollection"/>.
        /// </value>
        /// <remarks>
        /// <para>
        /// As the collection is empty and thread safe and synchronized this instance is also
        /// the <see cref="SyncRoot"/> object.
        /// </para>
        /// </remarks>
        public object SyncRoot
        {
            get { return this; }
        }

        /// <summary>
        /// Returns an enumerator that can iterate through a collection.
        /// </summary>
        /// <returns>
        /// An <see cref="IEnumerator"/> that can be used to 
        /// iterate through the collection.
        /// </returns>
        /// <remarks>
        /// <para>
        /// As the collection is empty a <see cref="NullEnumerator"/> is returned.
        /// </para>
        /// </remarks>
        IEnumerator IEnumerable.GetEnumerator()
        {
            return NullEnumerator.Instance;
        }

        /// <summary>
        /// Adds an element with the provided key and value to the 
        /// <see cref="EmptyDictionary" />.
        /// </summary>
        /// <param name="key">The <see cref="object" /> to use as the key of the element to add.</param>
        /// <param name="value">The <see cref="object" /> to use as the value of the element to add.</param>
        /// <remarks>
        /// <para>
        /// As the collection is empty no new values can be added. A <see cref="InvalidOperationException"/>
        /// is thrown if this method is called.
        /// </para>
        /// </remarks>
        /// <exception cref="InvalidOperationException">This dictionary is always empty and cannot be modified.</exception>
        public void Add(object key, object value)
        {
            throw new InvalidOperationException();
        }

        /// <summary>
        /// Removes all elements from the <see cref="EmptyDictionary" />.
        /// </summary>
        /// <remarks>
        /// <para>
        /// As the collection is empty no values can be removed. A <see cref="InvalidOperationException"/>
        /// is thrown if this method is called.
        /// </para>
        /// </remarks>
        /// <exception cref="InvalidOperationException">This dictionary is always empty and cannot be modified.</exception>
        public void Clear()
        {
            throw new InvalidOperationException();
        }

        /// <summary>
        /// Determines whether the <see cref="EmptyDictionary" /> contains an element 
        /// with the specified key.
        /// </summary>
        /// <param name="key">The key to locate in the <see cref="EmptyDictionary" />.</param>
        /// <returns><c>false</c></returns>
        /// <remarks>
        /// <para>
        /// As the collection is empty the <see cref="Contains"/> method always returns <c>false</c>.
        /// </para>
        /// </remarks>
        public bool Contains(object key)
        {
            return false;
        }

        /// <summary>
        /// Returns an enumerator that can iterate through a collection.
        /// </summary>
        /// <returns>
        /// An <see cref="IEnumerator"/> that can be used to 
        /// iterate through the collection.
        /// </returns>
        /// <remarks>
        /// <para>
        /// As the collection is empty a <see cref="NullEnumerator"/> is returned.
        /// </para>
        /// </remarks>
        public IDictionaryEnumerator GetEnumerator()
        {
            return NullDictionaryEnumerator.Instance;
        }

        /// <summary>
        /// Removes the element with the specified key from the <see cref="EmptyDictionary" />.
        /// </summary>
        /// <param name="key">The key of the element to remove.</param>
        /// <remarks>
        /// <para>
        /// As the collection is empty no values can be removed. A <see cref="InvalidOperationException"/>
        /// is thrown if this method is called.
        /// </para>
        /// </remarks>
        /// <exception cref="InvalidOperationException">This dictionary is always empty and cannot be modified.</exception>
        public void Remove(object key)
        {
            throw new InvalidOperationException();
        }

        /// <summary>
        /// Gets a value indicating whether the <see cref="EmptyDictionary" /> has a fixed size.
        /// </summary>
        /// <value><c>true</c></value>
        /// <remarks>
        /// <para>
        /// As the collection is empty <see cref="IsFixedSize"/> always returns <c>true</c>.
        /// </para>
        /// </remarks>
        public bool IsFixedSize
        {
            get { return true; }
        }

        /// <summary>
        /// Gets a value indicating whether the <see cref="EmptyDictionary" /> is read-only.
        /// </summary>
        /// <value><c>true</c></value>
        /// <remarks>
        /// <para>
        /// As the collection is empty <see cref="IsReadOnly"/> always returns <c>true</c>.
        /// </para>
        /// </remarks>
        public bool IsReadOnly
        {
            get	{ return true; }
        }

        /// <summary>
        /// Gets an <see cref="ICollection" /> containing the keys of the <see cref="EmptyDictionary" />.
        /// </summary>
        /// <value>An <see cref="ICollection" /> containing the keys of the <see cref="EmptyDictionary" />.</value>
        /// <remarks>
        /// <para>
        /// As the collection is empty a <see cref="EmptyCollection"/> is returned.
        /// </para>
        /// </remarks>
        public ICollection Keys
        {
            get { return EmptyCollection.Instance; }
        }

        /// <summary>
        /// Gets an <see cref="ICollection" /> containing the values of the <see cref="EmptyDictionary" />.
        /// </summary>
        /// <value>An <see cref="ICollection" /> containing the values of the <see cref="EmptyDictionary" />.</value>
        /// <remarks>
        /// <para>
        /// As the collection is empty a <see cref="EmptyCollection"/> is returned.
        /// </para>
        /// </remarks>
        public ICollection Values
        {
            get { return EmptyCollection.Instance; }
        }

        /// <summary>
        /// Gets or sets the element with the specified key.
        /// </summary>
        /// <param name="key">The key of the element to get or set.</param>
        /// <value><c>null</c></value>
        /// <remarks>
        /// <para>
        /// As the collection is empty no values can be looked up or stored. 
        /// If the index getter is called then <c>null</c> is returned.
        /// A <see cref="InvalidOperationException"/> is thrown if the setter is called.
        /// </para>
        /// </remarks>
        /// <exception cref="InvalidOperationException">This dictionary is always empty and cannot be modified.</exception>
        public object this[object key]
        {
            get { return null; }
            set { throw new InvalidOperationException(); }
        }

        /// <summary>
        /// The singleton instance of the empty dictionary.
        /// </summary>
        private static readonly EmptyDictionary s_instance = new EmptyDictionary();
    }
}
